Due to its strength, permanency, and relative low cost, concrete is the most important building material employed in modern construction. It is widely used for foundations of all types including buildings, bridges, dams, retaining walls, highways and the like. However, the adequacy of concrete for functioning under a particular set of conditions depends upon many factors such as the selection and mixing of materials, placement of the concrete and the structural design. One factor bearing upon the use of concrete is the fact that it is strong in compression, but relatively weak in tension. Therefore, structures in which concrete is likely to be in tension must be reinforced.
One method that is widely used to overcome the disadvantages of concrete's low tensile strength is the prestressing of concrete with reinforcement. This is accomplished by use of high strength steel wires, cables, or rods embedded within the concrete. In prestressing of concrete, the concrete member is precompressed and thus when the structure receives a load, the compression is relieved on that portion which would normally be in tension. Thus, for example, a beam is prestressed so that when the concrete is under load, the side normally in tension has no tensile forces acting upon it.
There are generally two methods of prestressing, namely--pretensioning and post-tensioning. Pretensioning consists of pouring concrete around steel members kept under tension until the concrete has gained sufficient strength. Tension is removed and compression forces are thereby imparted to the concrete through bonds between the steel members and the concrete. Post-tensioning consists of jacking bondfree cables against the ends of an already hardened concrete section and then anchoring the ends of the cable to maintain the cable in tension. In elongated concrete structures such as roads, highways, landing strips and the like, post-tensioning is used because it requires less concrete, and can be less expensive.
The configuration of an elongated slab such as a highway, however, presents special problems in the installation of the prestressing material. It has been proposed to prestress such elongated slabs from the sides of the slab. Examples of prestressing concrete structures from the sides are described in U.S. Pat. Nos. 2,590,685; 2,655,846; 2,833,186; 2,950,517; 3,072,994; 3,182,109; and 4,191,490. In some of these structures, the prestressing elements extend transverse to the length of the slab, and in others the prestressing elements extend diagonally across the slab. In each instance the prestressing elements are tensioned from the side of the slab and thus access must be provided for stressing equipment such as jacks, rachets or the like for use in tensioning the post-tensioning elements in the slab. Prestressing the slabs from the edges presents disadvantages in that the short transverse or diagonal elements require more labor to install. The necessity of providing access along the edges of the slab limits the use and prohibits the placement of slabs in a side-by-side abutting arrangement. In addition, these structures lack the substantial structural advantages present with longitudinally extending prestressing elements.
To avoid the disadvantages of prestressing from the sides of a slab transverse to its length, attempts have been made to run the prestressing elements longitudinally along the length of the slab. Examples of these attempts can be found in the U.S. Pat. Nos. 3,022,713; 3,089,215; 3,237,537; and 4,245,923.
In some of these prior art systems, concrete highway is poured in lengths of a few hundred feet with the prestressing elements therein extending longitudinally of the slab. A gap is left at the end of the slab and the ends of the prestressing elements extent into the gap. For example, in the patent to Stubbs, U.S. Pat. No. 3,089,215, slabs of concrete in a length of approximately 200 feet are poured with gaps in between the slabs. These gaps are of a sufficient size to allow the operation of post-tensioning jacks. Once the concrete is set and the post-tensioning has been completed, concrete is poured into these gaps to the final level of the elongated slabs. The patent to Stubbs acknowledges that these closure strips do not have the strength or flexibility of the adjacent prestressed slabs. Constructing post-tension slabs by providing gaps between the slabs also presents disadvantages in that the pavement cannot be used during the curing of the slab.